The present invention relates to transmissions of motor vehicles, and more specifically to transmission cases of automatic transaxles.
FIGS. 3 and 4 show a conventional automatic transaxle (4HP14Q type transaxle by ZF company). This transaxle includes a torque converter housing a, a transmission case b, a side cover c, a torque converter d, and a differential e. The transmission has one multiple disc clutch near the side cover c within the transmission case b. This clutch has a piston f and an oil chamber g for applying pressure on the piston f. A bolt h is one of fasteners which structurally connect the side cover c to the transmission case b. The transmission case b is formed with an internally threaded hole i for receiving the bolt h.
FIG. 4 shows a casting defect j, such as a shrinkage cavity, hole or crack, or a blowhole, which is formed in the transmission case b during the process of casting metal into the transmission case b. In this transmission case b, the oil chamber g is very close to the threaded hole i. Therefore, the defect j is easily made open in the surfaces by a subsequent machining process for forming the threaded hole i and the inside surface of the oil chamber g, and the oil chamber g is easily connected with the hole i by the defect j. As a result, the oil pressure leaks from the oil chamber g through the defect j, and the clutch becomes unable to function properly.
The threaded hole i of this example is a blind hole which is closed at one end, so that trimming is required in the process for forming the threaded hole i.